The invention relates to an optical communication unit, and more particularly, to an optical communication unit suited for time division bidirectional communication and the like in the terminals for subscriber-system communication using optical fibers.
As shown in FIG. 4, the basic construction of a bidirectional optical communication unit comprises a light emitting element 21 such as a semiconductor laser that generates transmission signal light, a light receiving element 22 comprising a photo diode, photo transistor, photo cell, and the like, which receives the receiving signal light via a half mirror 23, a condenser lens 24 that joins the transmission signal light to an optical transmission path such as optical fibers, and a rod lens 25 for joining the condensed light to a ferrule 26 of the optical transmission path, and the rod lens 25 and ferrule 26 are held with a split sleeve 28 so that the center of the rod lens 25 aligns with the center of the fiber 27 at the center of the ferrule 26. The contact surfaces of the rod lens 25 and ferrule 26 are ground to a convex sphere so that they come into physical contact. The rod lens 25 is in contact with the tip end of the optical fiber, where physical contact means that the contact surfaces closely stick together by polishing the contact surfaces of lens or ferrule end face to a convex sphere so as to prevent Fresnel reflection.
In this optical communication unit, the transmission signal light and the receiving signal light are separated by inserting the half mirror 23 into an optical transmission path, the light emitting element 21 and the light receiving element 22 are arranged separated from each other, and the transmission signal light is condensed by the light condenser lens 24 and converged to the physical contact section between the rod lens 25 and the ferrule 26. The receiving signal light from the optical fiber is 50% reflected by the half mirror and received by the light receiving element 22. The end face of the light emitting element 21 and the light receiving surface of the light receiving element 22 are tilted (or inclined) to the optical axis to prevent the receiving signal light, which is reflected on the surface of the light receiving element 22, or is transmitted through the half mirror 23 and reflected at the end face of the light emitting element 21, returning to the optical fiber, and serving as a noise source.
A conventional optical communication unit is designed with the surface of the light emitting element or the light receiving element tilted obliquely to prevent the receiving signal light from the optical transmission path from being reflected at the light emitting element or light receiving element and from returning to the optical transmission path, and then from returning to the light emitting source of the signal light to generate noise. Consequently, the outgoing beam from the light emitting element for transmission is not coupled with the condenser lens 24 by only one half, which is further reduced in half at the half-mirror 23 and is incident on the optical transmission path. Since the light incident on the optical fiber 27 is further reduced, the coupling efficiency of the light emitting element 21 and the condenser lens 24 lowers, which requires increased output of the light emitting element, causing a serious problem.
On the other hand, in the Japanese Unexamined Patent Publication No. 77382/1986, for a semiconductor laser used for a light emitting element such as optical communication units and the like, there is disclosed a semiconductor laser with a construction in which a semiconductor laser chip 31 is disposed obliquely to the condenser lens, almost all the transmission signal light is coupled with the condenser lens, and the signal receiving light from the optical transmission path and incident on the light emitting element side has the light emitting element end face slanted to the optical axis of the condenser lens by setting the tilting angle .tau. to more than 5.degree. to perpendicular of both end faces of the stripe groove 32, a light emitting area, and deviating the direction of outgoing beam by a specified angle perpendicular to the chip end face of the light emitting element, as shown in FIG. 5, thereby enabling the receiving signal light to be reflected in directions not related to the optical transmission path. However, when an optical transmission unit configured as above is assembled using the semiconductor laser with the above construction, it must be assembled in such a manner that the outgoing beam direction of the light emitting element conforms to the optical axis direction of the condenser lens, but there is no positioning reference between the outgoing beam direction of the light emitting element and the shape of the mounted plate, and it is impossible to completely align the outgoing beam direction with the optical axis of the condenser lens, creating a problem of lowering the coupling efficiency.